Van Leeuwenhoek never wrote books, just letters. In his youth he was apprenticed to a draper; a later Civil position allowed him to devote time to his hobby: grinding lenses and using them to study tiny objects. With his simple microscopes — skillfully ground, powerful single lenses capable of high image quality — he observed protozoa in rainwater and pond and well water and bacteria in the human mouth and intestine. He also discovered blood corpuscles, capillaries, and the structure of muscles and nerves. In 1677, he first described the spermatozoa of insects, dogs, and humans. His research on lower animals argued against the doctrine of spontaneous generation, and his observations helped lay the foundations for the sciences of bacteriology and protozoology.